<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
  <input type="text" id="opt1" value="">+
  <input type="text" id="opt2" value="">
  <button>=</button>
  <input type="text" id="opt3" value="">
</body>
<script>
  var oPt1 = document.querySelector("#opt1")
  var oPt2 = document.querySelector("#opt2")
  var oPt3 = document.querySelector("#opt3")
  var oBtn = document.querySelector('button')
oBtn.onclick = function(){
    oPt3.value=oPt1.value*1 + oPt2.value*1
}
</script>
</html>